Physics Tip Sheet #35 - July 11, 2003

Highlights of this tip sheet include heartbeat analysis to predict mortality rate, a new type of superconductor, growing nanobubbles and clouds, and new research on carbon nanotube transistors suggesting that size doesn't matter.

A new paper has added diagnostic capability to an earlier study of heartbeat analysis among congestive heart failure patients. The original 2002 paper, also published in Physical Review E, suggested that the heartbeat patterns of healthy patients were significantly different from those of patients with congestive heart failure, but the study lacked the relevant clinical data needed to give it diagnostic importance. Clinical data has since been made available to the researchers, who have published an addendum to the original paper, giving it a firmer theoretical basis and claiming that the heartbeat patterns of patients with congestive heart failure can be used to determine their mortality risk.

By taking conventional superconducting materials, smashing them into little pieces, gluing them back together and then cooking them, researchers have found a new type of superconductor that carries more current and remains superconducting in a much higher magnetic field than conventional superconductors. The researchers speculate the new superconducting materials can be used to make magnets that produce higher magnetic fields and MRI scanners will be able to use them to produce a sharper picture.

The growth of water droplets to form clouds belongs to the same class of phenomena as the separation of alloy components upon ageing; the decomposition of binary mixed fluids; and the formation of bubbles from a supersaturated or superheated solution. In all cases, the growth rate is limited by the surface energy of the droplet. The authors have developed a new computer simulation to evaluate the surface tension of supersaturated interfaces and showed that the supersaturation of the atmosphere decreases the surface tension from its usual coexistence value.

Smaller transistors usually perform better, a fact that has driven the semiconductor industry to shrink the size of silicon transistors by a factor of two approximately every three years. It is anticipated that in about 15 years, they will have become too small to be turned off. Carbon nanotubes are the leading candidate to replace silicon, having already been shown to carry more current per unit of width. It has so far been difficult to say what happens when they are made shorter, because the switching action in a nanotube transistor depends on the properties of the metal contacts rather than on the nanotube itself. This paper describes a new type of carbon nanotube transistor in which the flow of current in the nanotube can be switched on and off independently of the metal-nanotube contacts, allowing the researchers to study the properties of different length transistors. They found the current in the nanotube was the same regardless of length, meaning that electrons move along the nanotube channel unimpeded. This explains why nanotube transistors perform so well and suggests that, for nanotubes, size doesn't matter.